The influence of water activity (a(w)) on the kinetics of aflatoxin and zearalenone production in amaranth grains at 25 degrees C was studied. Minimum a(w) for aflatoxin production in this substrate was 0.825. Accumulation of the four aflatoxins (B1, B2, G1 and G2) was similar at a(w) 0.825 (maximum 81.2 microg/kg after 42 days) and 0.868 (maximum 109.6 microg/kg after 49 days). Maximum accumulation of total aflatoxins at a(w) 0.902 (260.4 microg/kg) was detected after 21 days, with an appreciable increment in the concentration of aflatoxins B1 and G1. These quantities were lower than those reported for aflatoxin production on other cereals and legumes, indicating that amaranth is not a good substrate for aflatoxin production. Zearalenone was not detected at a(w) 0.902. Maximum accumulation of zearalenone was 1.5 microg/g after 35 days at a(w) 0.925 and 11.1 microg/g after 49 days at a(w) 0.950.